Metal-poor stars in the Galactic bulge - the oldest in the Milky Way?

by Louise Howes (Lund University)

Friday 7 Oct 2016, 10:30 → 11:30 Europe/Stockholm

FC61

Over the past five years, the old assumption that metal-poor stars exist only in the Galactic halo has been shown to be false. Several observational campaigns have succeeded in finding very metal-poor giant stars within the confines of the Galactic bulge, and following the principle that the Milky Way formed "inside-out", there is significant theoretical weight behind the idea that these stars are the oldest in the Galaxy. By studying the chemistry of these stars, we can gain insight into the earliest stages of the Milky Way’s formation, including what the very first stars of the Galaxy would have looked like. In this seminar I will present the latest findings of the EMBLA survey, which has successfully identified more than 800 RGB stars in the bulge with [Fe/H]<-2. The survey has observed 50 with high-resolution spectrographs, and has found some peculiar chemical differences in them compared to the younger metal-poor stars found in the halo. This includes a lack of stars with the large carbon enhancement that is characteristic of the lowest metallicity objects in the halo. We have also been able to verify — for a small subsample of our stars — that the majority are indeed on tightly-bound orbits, rather than passing through the bulge region on typical eccentric halo star orbits. This discovery confirms that these stars live permanently in the bulge, and that the bulge we see today has grown around them. We are following up some 200 of these stars with Kepler K2 Campaign 9, from which we will get highly accurate stellar parameters, and hopefully be able to derive a statistical age. We hope to then confirm that we have truly found the remnants of the first stars of the Milky Way.